Rubber composition for bead apex and tire having bead apex prepared using same

ABSTRACT

There is provided a rubber composition for a bead apex which can enhance extrusion processability by optimizing a rate of vulcanization during a vulcanization process, and can improve rigidity, steering stability and a low fuel consumption property, a tire having a bead apex prepared by using the rubber composition for a bead apex and being capable of enhancing steering stability and reducing rolling resistance and a tire for a sports utility vehicle (SUV) having further enhanced durability. The rubber composition for a bead apex comprises a diene rubber, a phenol resin and/or a modified phenol resin, sulfur, hexamethylenetetramine, a vulcanization accelerator, and at least one kind of vulcanization acceleration auxiliary selected from the group consisting of a citraconimide compound, a condensate of alkyl phenol and sulfur chloride, an organic thiosulfate compound and a compound represented by the general formula: R 1 —S—S-A-S—S—R 2 .

CROSS REFERENCE WITH PCT APP

The present application is a 37 C.F.R. §1.53(b) divisional of, andclaims priority to, U.S. application Ser. No. 12/226,572 filed Oct. 22,2008. Application Ser. No. 12/226,572 is the national phase under 35U.S.C. §371 of International Application No. PCT/JP2007/062990, filed onJun. 28, 2007. Priority is also claimed to Japanese Application No.2006-190819 filed on Jul. 11, 2006. The entire contents of each of theseapplications is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a rubber composition for a bead apexand a tire having a bead apex prepared by using the rubber composition.

BACKGROUND ART

For a rubber composition for a bead apex of a tire, so far emphasis hasbeen placed only on increasing a complex modulus (E*) in order toenhance steering stability.

For increasing the complex modulus (E*), there is, for example, a methodof using a carbon black exhibiting a large reinforcing effect in a largeamount, however, the method has a problem that tan δ is increased. Onthe other hand, in order to lower tan δ, for example, there is a methodof increasing a content of a vulcanizing agent such as sulfur. However,in the case where a large amount of vulcanizing agent is used, partialscorch of the rubber occurs easily during an extruding process due to anaccelerated initial vulcanization rate. The initial vulcanization ratemay be somewhat slowed down by compounding a specified amount of asynthetic rubber, and if a retarder PVI is used, processing can becarried out. Even in that case, however, productivity still remains at alow level. In addition, when the retarder PVI is added in an amountexceeding 1.0 part by weight based on 100 parts by weight of a rubbercomponent, blooming thereof occurs during processing of the rubber, andtherefore there is a limit in improvement in a vulcanization rate.

JP2002-36832A discloses a rubber composition for a bead apex whichcomprises specified amounts of a rubber component and a citraconimidecompound. However, the rubber composition does not comprise a phenolresin and/or a modified phenol resin, and has a problem that a complexmodulus (E*) of not less than 10 MPa cannot be obtained, and steeringstability and responsiveness to steering are insufficient.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a rubber compositionfor a bead apex which can enhance extrusion processability by optimizinga rate of vulcanization during a vulcanization process, and improverigidity, steering stability and low fuel consumption property, andanother object of the present invention is to provide a tire having abead apex prepared by using the rubber composition for a bead apex andbeing capable of enhancing steering stability and reducing rollingresistance and a tire for a sports utility vehicle (SUV) having furtherenhanced durability, in which a ratio of a height of the bead apex to aheight of a tire section is higher as compared with a tire for apassenger car.

The present invention relates to a rubber composition for a bead apexcomprising 5 to 25 parts by weight of a phenol resin and/or a modifiedphenol resin, 5.1 to 7.0 parts by weight of sulfur, 0.5 to 2.5 parts byweight of hexamethylenetetramine, 2.0 to 5.0 parts by weight of asulfenamide vulcanization accelerator and/or a thiazole vulcanizationaccelerator, and 0.1 to 5 parts by weight of at least one vulcanizationacceleration auxiliary selected from the group consisting of acitraconimide compound, a condensate of alkyl phenol and sulfurchloride, an organic thiosulfate compound and a compound represented bythe following general formula:

R¹—S—S-A-S—S—R²

wherein A represents an alkylene group having 2 to 10 carbon atoms, andeach of R¹ and R² independently represents a monovalent organic grouphaving a nitrogen atom, based on 100 parts by weight of a diene rubber.

In the above-mentioned rubber composition for a bead apex, the totalamount of hexamethylenetetramine and sulfenamide vulcanizationaccelerator and/or thiazole vulcanization accelerator is preferably 3.5to 7.5 parts by weight based on 100 parts by weight of the diene rubber.

In the above-mentioned rubber composition for a bead apex, thevulcanization acceleration auxiliary is preferably a citraconimidecompound and/or a condensate of alkyl phenol and sulfur chloriderepresented by the following chemical formula:

wherein n is 0 or an integer of 1 to 10, X is an integer of 2 to 4, R isan alkyl group having 5 to 12 carbon atoms.

In the above-mentioned rubber composition for a bead apex, thecitraconimide compound is preferably 1,3-bis(citraconimidemethyl)benzenerepresented by the following chemical formula.

Also, the present invention relates to a tire having a bead apexprepared by using the above-mentioned rubber composition for a beadapex.

Further, the present invention relates to a tire for a sports utilityvehicle having a bead apex prepared by using the above-mentioned rubbercomposition for a bead apex.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cross-sectional view of the tire for a passenger carhaving the bead apex of the present invention.

FIG. 2 is a partial cross-sectional view of the tire for a sportsutility vehicle (SUV) having the bead apex of the present invention.

FIG. 3 is a diagrammatic view showing a ratio of a height of the beadapex of the present invention to a height of a section in a tire for apassenger car.

FIG. 4 is a diagrammatic view showing a ratio of a height of a bead apexof the present invention to a height of a section in a tire for a sportsutility vehicle (SUV).

FIG. 5 is a partial cross-sectional view of a tire for a passenger carhaving a strip apex prepared by using the rubber composition for a beadapex of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a partial cross-sectional view of the tire for a passenger carhaving the bead apex of the present invention. In the present invention,unless otherwise specified, a tire means a tire for a passenger car anda sports utility vehicle (SUV).

As shown in FIG. 1, the tire of the present invention comprises tiremembers of a passenger car, for example, a sidewall 1, a tire clinch 2,a bead core 3, a bead apex 4, an inner liner 5, a belt 6 and a tread 7.In FIG. 1, the bead apex 4 is arranged inside the tire clinch 2 andextends outwardly from the bead core 3 in the radial direction. Thesidewall 1 as an outer surface of the tire is so arranged as to extendinwardly from the both ends of the tread 7 in the radial direction ofthe tire. The tire clinch 2 is arranged at inner ends of each sidewall1. The inner end of the tire in its radial direction is so designed asto be able to contact with a metallic rim 8.

FIG. 2 is a partial cross-sectional view of a tire for a sports utilityvehicle (SUV) having the bead apex of the present invention.

As shown in FIG. 2, the tire for a sports utility vehicle (SUV) of thepresent invention is comprised of tire members such as a sidewall 11, atire clinch 12, a bead core 13, a bead apex 14, an inner liner 15, abelt 16 and a tread 17. A general embodiment of the tire is as explainedin FIG. 1. In addition, the inner end of the tire for a sports utilityvehicle (SUV) in its radial direction is so designed as to be able tocontact with a metallic rim 18.

As shown in FIG. 2, in a tire for a sports utility vehicle (SUV), thesidewall 11 and the bead apex 14 occupy a high volume percentage ascompared with other tire members, and therefore characteristics of thesidewall 11 and the bead apex 14 as the tire members have a large effecton characteristics of a molded tire.

In tires for a sports utility vehicle (SUV), shock absorptivity andresponsiveness to quick steering for turning during running on a roughroad are expected, and on the other hand, characteristics such assatisfactory ride quality on a flat road and smooth ride quality atstarting of running (free from flat spot vibration) under cold conditionare demanded. Accordingly, the sidewall 11 and the bead apex 14 of thetire for a sports utility vehicle (SUV) are required to havecharacteristics such as early recovery from permanent set (for example,permanent set under cold condition) (for example, usually being capableof recovering in about 30 minutes at a tire temperature of 70° C.) andresponsiveness to quick steering for turning (for example, high rigidity(E*)).

The rubber composition for a bead apex of the present inventioncomprises a diene rubber, a phenol resin and/or a modified phenol resin,sulfur, a vulcanization accelerator, and a vulcanization accelerationauxiliary.

Examples of the diene rubber are rubber components such as a naturalrubber (NR), isoprene rubber (IR), butadiene rubber (BR),styrene-butadiene rubber (SBR), butyl rubber (IIR), halogenated butylrubber (X-IIR), chloroprene rubber (CR), and acrylonitrile-butadienerubber (NBR). These diene rubbers are not particularly limited and maybe used alone or two or more kinds thereof may be used in combination.

The phenol resin is not particularly limited and examples of the phenolresin are those prepared by reacting phenols with aldehydes such asformaldehyde, acetaldehyde, and furfural in the presence of an acidcatalyst or an alkaline catalyst.

Examples of the modified phenol resin are phenol resins modified, forexample, with cashew oil, tall oil, linseed oil, various animal andvegetable oils, unsaturated fatty acid, rosin, an alkylbenzene resin,aniline, and melamine.

As the phenol resin or the modified phenol resin, in view of theircapability of raising hardness (Hs), the modified phenol resins arepreferable, and the phenol resins modified with cashew oil or the phenolresins modified with rosin are preferable.

A content of the phenol resin and/or the modified phenol resin is notless than 5 parts by weight, preferably not less than 7 parts by weightbased on 100 parts by weight of the diene rubber. In the case where thecontent of the phenol resin and/or the modified phenol resin is lessthan 5 parts by weight, sufficient Hs cannot be obtained. At the sametime, the content of the phenol resin and/or the modified phenol resinis not more than 25 parts by weight, preferably not more than 20 partsby weight. In the case where the content of the phenol resin and/or themodified phenol resin is more than 25 parts by weight, strength at breakdecreases.

With respect to sulfur used in the present invention, insoluble sulfuris preferable because blooming thereof during processing can beinhibited and its dispersibility is superior, and exemplified areCrystex HSOT20 available from Flexsys Kabushiki Kaisha or Sanfel EXavailable from SANSHIN CHEMICAL INDUSTRY CO., LTD.

While insoluble sulfur represents sulfur being insoluble, for example,in carbon disulfide, rubber-like hydrocarbon or the like, the insolublesulfur used in the present invention is high molecular weight sulfurcontaining components being insoluble particularly in carbon disulfidein a ratio of not less than 80%. In addition, the content of thecomponents being insoluble in carbon disulfide may be not less than 90%.Sulfur containing insoluble components in carbon disulfide in a ratio ofnot less than 60% is not preferable in the present invention becauseblooming occurs when such sulfur is compounded in an amount of not lessthan 3 parts by weight based on 100 parts by weight of the diene rubber.

The content of sulfur is not less than 5.1 parts by weight, preferablynot less than 5.25 parts by weight based on 100 parts by weight of thediene rubber. In the case where the content of sulfur is less than 5.1parts by weight, sufficient hardness (Hs) cannot be obtained. At thesame time, the content of sulfur is not more than 7.0 parts by weight,preferably not more than 6.5 parts by weight. In the case where thecontent of sulfur is more than 7.0 parts by weight, blooming thereofoccurs during processing, thereby deteriorating stickiness of therubber, and as a result, during molding of a tire, the bead apex doesnot adhere to the neighboring inner liner (case) and bubbles (porosity)are generated inside the tire. In addition, even during vulcanization,blooming occurs on the tire surface, which causes uneven distribution ofhardness (Hs) of the vulcanized rubber. In the case where the insolublesulfur is compounded, the content of sulfur as used herein represents asulfur content excluding oil content in the insoluble sulfur.

The rubber composition contains, as the vulcanization accelerators,hexamethylenetetramine (HMT) and a sulfenamide vulcanization acceleratorand/or a thiazole vulcanization accelerator.

A content of hexamethylenetetramine (HMT) is not less than 0.5 part byweight, preferably not less than 1.0 part by weight based on 100 partsby weight of the diene rubber. In the case where the content of HMT isless than 0.5 part by weight, Hs is decreased due to an insufficientamount of generated methylene and insufficient crosslinking reaction ofphenol. At the same time, the content of HMT is not more than 2.5 partsby weight, preferably not more than 2.0 parts by weight. In the casewhere the content of HMT is more than 2.5 parts by weight, crosslinkingreaction of phenol is saturated and ammonia as a by-product causespartial scorch of rubber (shortening of scorch time).

In the present invention, partial scorch of the rubber means that whenthe rubber is kneaded in a Banbury mixer and then subjected to extrudingand mold-processing, lumps of partially scorched rubber are partlygenerated in the extruded rubber or partially scorched rubbers arecollected in a cylinder of an extruder and stick thereto. The scorchtime represents a period of time (referred to as T₁₀ in the presentinvention) until a stress torque of the rubber is elevated by 10% undera given temperature (for example, 130° C. in the present invention).Between the scorch time and the partial scorch of the rubber, there issuch a relation that when the scorch time is short, partial scorch ofthe rubber occurs easily.

Examples of the sulfenamide vulcanization accelerator are, for example,N-tert-butyl-2-benzothiazolylsulfenamide (e.g., NOCCELER NS availablefrom Ouchi Shinko Chemical Industrial Co., Ltd.),N,N′-dicyclohexyl-2-benzothiazolylsulfenamide (e.g., NOCCELER DZavailable from Ouchi Shinko Chemical Industrial Co., Ltd.),N-cyclohexyl-2-benzothiazolylsulfenamide (e.g., NOCCELER CZ availablefrom Ouchi Shinko Chemical Industrial Co., Ltd.),N-oxydiethylene-2-benzothiazolylsulfenameide (e.g., NOCCELER MSA-Gavailable from Ouchi Shinko Chemical Industrial Co., Ltd.), andN,N-diisopropyl-2-benzothiazolylsulfenamide (DPBS). From the viewpointof stable scorching and superior physical properties of vulcanizate,N-tert-butyl-2-benzothiazolylsulfenamide andN,N′-dicyclohexyl-2-benzothiazolylsulfenamide are preferable.

Examples of the thiazole vulcanization accelerator are, for instance,2-mercaptobenzothiazole (e.g., NOCCELER M available from Ouchi ShinkoChemical Industrial Co., Ltd.), di-2-benzothiazolyldisulfide (e.g.,NOCCELER DM available from Ouchi Shinko Chemical Industrial Co., Ltd.),2-(4′-morpholinodithio)benzothiazole (e.g., NOCCELER MDB available fromOuchi Shinko Chemical Industrial Co., Ltd.), and2-(N,N-diethylthiocarbamoylthio)benzothiazole (e.g., NOCCELER 64available from Ouchi Shinko Chemical Industrial Co., Ltd.). Among them,from the viewpoint of capability of obtaining an appropriate rate ofrubber vulcanization, 2-mercaptobenzothiazole is preferable.

A content of the sulfenamide vulcanization accelerator and/or thethiazole vulcanization accelerator is not less than 2.0 parts by weight,preferably not less than 2.5 parts by weight based on 100 parts byweight of the diene rubber. In the case where the content of thesulfenamide vulcanization accelerator and/or the thiazole vulcanizationaccelerator is less than 2.0 parts by weight, sufficient Hs cannot beobtained. At the same time, the content of the sulfenamide vulcanizationaccelerator and/or the thiazole vulcanization accelerator is not morethan 5.0 parts by weight, preferably not more than 4.5 parts by weight.In the case where the content of the sulfenamide vulcanizationaccelerator and/or the thiazole vulcanization accelerator is more than5.0 parts by weight, scorch time is excessively shortened and partialscorch of the rubber is caused.

The total content of hexamethylenetetramine (HMT) and sulfenamidevulcanization accelerator and/or thiazole vulcanization accelerator ispreferably not less than 3.5 parts by weight, more preferably not lessthan 4.0 parts by weight, further preferably not less than 5.0 parts byweight based on 100 parts by weight of the diene rubber so that anecessary hardness (Hs) can be exhibited. In addition, the total contentof hexamethylenetetramine (HMT) and sulfenamide vulcanizationaccelerator and/or thiazole vulcanization accelerator is preferably notmore than 7.5 parts by weight, more preferably not more than 7.0 partsby weight based on 100 parts by weight of the diene rubber so that thescorch time can be a processing limit (namely, the scorch time can bemade longer).

There are vulcanization accelerators other than HMT, the sulfenamidevulcanization accelerator and the thiazole vulcanization accelerator,for example, guanidine vulcanization accelerators such asdiphenylguanidine, hexamethoxymethylolmelamine (HMMM), andhexamethoxymethylol pentamethyl ether (HMMPME). However, HMT and thesulfenamide vulcanization accelerator and/or the thiazole vulcanizationaccelerator are most suitable since both of the crosslinking of a phenolresin and sulfur vulcanization can be carried out and an adequateprocessability (adequate scorch time) can be obtained.

In the present invention, there is contained, as the vulcanizationacceleration auxiliary, at least one kind of compound selected from thegroup consisting of a citraconimide compound, a condensate of alkylphenol and sulfur chloride, an organic thiosulfate compound, and acompound represented by the following general formula:

R¹—S—S-A-S—S—R²

wherein A represents an alkylene group having 2 to 10 carbon atoms, andeach of R¹ and R² independently represents a monovalent organic grouphaving a nitrogen atom.

For the reason that the scorch time is not affected, it is preferable touse the citraconimide compound as the vulcanization accelerationauxiliary.

Preferable as the citraconimide compound are bis-citraconimides becauseof their favorable properties such as excellent thermal stability anddispersibility in a rubber. More specifically, examples of thecitraconimide compound are 1,2-bis(citraconimidemethyl)benzene,1,3-bis(citraconimidemethyl)benzene,1,4-bis(citraconimidemethyl)benzene,1,6-bis(citraconimidemethyl)benzene,2,3-bis(citraconimidemethyl)toluene,2,4-bis(citraconimidemethyl)toluene,2,5-bis(citraconimidemethyl)toluene,2,6-bis(citraconimidemethyl)toluene, 1,2-bis(citraconimideethyl)benzene,1,3-bis(citraconimideethyl)benzene, 1,4-bis(citraconimideethyl)benzene,1,6-bis(citraconimideethyl)benzene, 2,3-bis(citraconimideethyl)toluene,2,4-bis(citraconimideethyl)toluene, 2,5-bis(citraconimideethyl)toluene,2,6-bis(citraconimideethyl)toluene and the like. Among them,1,3-bis(citraconimidemethyl)benzene is preferable because its thermalstability is especially stable, its dispersibility in the rubber isparticularly excellent and a rubber composition having a high hardness(Hs) can be obtained (control of reversion).

1,3-bis(citraconimidemethyl)benzene is represented by the followingchemical formula:

For the reason that a rubber composition having a higher hardness (Hs)can be obtained, it is preferable to use a condensate of alkyl phenoland sulfur chloride as the vulcanization acceleration auxiliary.

The condensate of alkyl phenol and sulfur chloride is represented by thefollowing chemical formula:

wherein n is 0 or an integer of 1 to 10, X is an integer of 2 to 4, R isan alkyl group having 5 to 12 carbon atoms.

For the reason that dispersibility of the condensate of alkyl phenol andsulfur chloride in the rubber is satisfactory, n is preferably 0 or aninteger of 1 to 10, more preferably an integer of 1 to 9.

For the reason that a high hardness can be exhibited efficiently(control of reversion), preferably X is an integer of 2 to 4, morepreferably X is 2. In the case where X is more than 4, the condensatetends to be thermally unstable. In the case where X is 1, the sulfurcontent (weight of sulfur) in the condensate of alkyl phenol and sulfurchloride is small.

For the reason that dispersibility of the condensate of alkyl phenol andsulfur chloride in the rubber is satisfactory, R is preferably an alkylgroup having 5 to 12 carbon atoms, more preferably an alkyl group having6 to 9 carbon atoms.

Example of the condensate of alkyl phenol and sulfur chloride is TACKROLV200 being available from TAOKA CHEMICAL CO., LTD., having a sulfurcontent of 24% by weight and represented by the following formula:

in which n is 0 to 10, X is 2 and R is an alkyl group of C₈H₁₇.

For the reason that a high hardness (Hs) can be obtained (control ofreversion), it is preferable to use the organic thiosulfate compoundrepresented by the following formula as the vulcanization accelerationauxiliary.

The organic thiosulfate compound is represented by the following generalformula:

MO₃S—S—(CH₂)_(m)—S—SO₃M

wherein m is an integer of 3 to 10, and M represents lithium, potassium,sodium, magnesium, calcium, barium, zinc, nickel, or cobalt, and thecompound is hydrophilic and may contain crystal water.

m is preferably an integer of 3 to 10, more preferably an integer of 3to 6. In the case where m is 2 or less, there is a tendency thatadequate thermal fatigue resistance cannot be obtained. On the otherhand, in the case where m is 11 or more, improvement in thermal fatigueresistance tends to be relatively small though a molecular weightincreases.

M is preferably lithium, potassium, sodium, magnesium, calcium, barium,zinc, nickel, or cobalt, and potassium or sodium is more preferable.

In addition, the compound may contain crystal water in its molecule.

Examples of the organic thiosulfate compound which can be used in thepresent invention are sodium salt-monohydrate, sodium salt-dihydrate andthe like from the viewpoint that hydrates are stable at normaltemperature under normal pressure. For economical reason that sodiumchloride is cheap, a derivative of sodium thiosulfate, e.g.,1,6-hexamethylene sodium dithiosulfate dihydrate is preferable.

The 1,6-hexamethylene sodium dithiosulfate dihydrate is represented bythe following chemical formula:

Besides the aforementioned three vulcanization acceleration auxiliaries,it is preferable to use a compound represented by the following formulaas the vulcanization acceleration auxiliary usable in the presentinvention:

R¹—S—S-A-S—S—R²

wherein A represents an alkylene group having 2 to 10 carbon atoms, andeach of R¹ and R² independently represents a monovalent organic grouphaving a nitrogen atom, for the reason that the auxiliary can bedispersed satisfactorily in the rubber and sulfur can be introduced, andhybrid crosslinking can be formed by introducing the auxiliary in themidst of the -Sx- crosslinking of the condensate of alkyl phenol andsulfur chloride.

A is preferably an alkylene group. The alkylene group may be a linear,branched, or cyclic one and any one can be used without particularlimitation, and the linear alkylene group is preferable.

The number of carbon atoms of A is preferably 2 to 10, more preferably 4to 8. In the case where the number of carbon atoms of A is 1 or less,there is a tendency that thermal stability is poor and the merits of S—Sbonds cannot be obtained. On the contrary, in the case where the numberof carbon atoms of A is 11 or more, a distance between the polymersbonded through the vulcanization acceleration auxiliary will becomelonger than crosslinked chains of S₈ and there is a tendency that asubstitution of -Sx- with the vulcanization acceleration auxiliary isdifficult, crosslinking does not proceed and an effect of adding thevulcanization acceleration auxiliary cannot be obtained. Here x of -Sx-means an integer of not more than 8 (namely, 1 to 8).

Examples of the alkylene group (A) satisfying the above conditions arean ethylene group, a trimethylene group, a tetramethylene group, apentamethylene group, a hexamethylene group, a heptamethylene group, anoctamethylene group, and a decamethylene group. Among them, thehexamethylene group is preferable as the alkylene group (A) because ofsmooth substitution for Sx (x=2 to 8) in the sulfur vulcanizationbetween polymer/vulcanization acceleration auxiliary/polymer andsuperior thermal stability.

It is preferable that each of R¹ and R² is independently a monovalentorganic group having a nitrogen atom, more preferably one having atleast one aromatic ring, further preferably one having a bondrepresented by the formula ═N—C(═S)— formed by bonding a carbon atom toa dithio group.

Each of R¹ and R² may be the same or different, and because of easypreparation, R¹ and R² are preferably the same.

Examples of the compound satisfying the above requirements are, forinstance,

-   1,2-bis(N,N′-dibenzylthiocarbamoyldithio)ethane,-   1,3-bis(N,N′-dibenzylthiocarbamoyldithio)propane,-   1,4-bis(N,N′-dibenzylthiocarbamoyldithio)butane,-   1,5-bis(N,N′-dibenzylthiocarbamoyldithio)pentane,-   1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane,-   1,7-bis(N,N′-dibenzylthiocarbamoyldithio)heptane,-   1,8-bis(N,N′-dibenzylthiocarbamoyldithio)octane,-   1,9-bis(N,N′-dibenzylthiocarbamoyldithio)nonane, and-   1,10-bis(N,N′-dibenzylthiocarbamoyldithio)decane. Among these,    1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane is preferable    because of its superior thermal stability and excellent    dispersibility in the rubber.

Example of a commercially available compound represented by the formula:

R¹—S—S-A-S—S—R²

wherein A represents an alkylene group having 2 to 10 carbon atoms, andeach of R¹ and R² independently represents a monovalent organic grouphaving a nitrogen atom, is, for instance, VULCUREN TRIAL PRODUCT KA9188(1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane) available from BAYER.

Among the aforementioned four kinds of vulcanization accelerationauxiliaries, 1,3-bis(citraconimidemethyl)benzene is preferable becauseof its favorable thermal stability since no sulfur is contained in itsmolecular structure and for the reason that sulfur is not releasedduring crosslinking and the initial vulcanization rate is notexcessively high. In addition, a condensate of alkyl phenol and sulfurchloride is preferable for the reasons that dispersion thereof in therubber composition is satisfactory by effects of a benzene ring andC₈H₁₇ branched chain (alkyl group), sulfur can be released and an ultrahigh hardness (Hs) can be obtained.

A content of the vulcanization acceleration auxiliary is not less than0.1 part by weight, preferably not less than 0.25 part by weight basedon 100 parts by weight of the diene rubber. In the case where thecontent of the vulcanization acceleration auxiliary is less than 0.1part by weight, sufficient hardness cannot be obtained. At the sametime, the content of the vulcanization acceleration auxiliary is notmore than 5 parts by weight, preferably not more than 4 parts by weight.In the case where the content of the vulcanization accelerationauxiliary is more than 5 parts by weight, boosting of the vulcanizationaccelerator occurs, resulting in accelerating the crosslinking of therubber, making a crosslinked network dense, and saturating an effect ofincreasing hardness. As a result, hardness is not increased and only thescorch time becomes short.

In the case where a citraconimide compound, an organic thiosulfatecompound, or a compound represented by the formula R¹—S—S-A-S—S—R² iscontained as the vulcanization acceleration auxiliary, the contentthereof is preferably 0.5 to 5.0 parts by weight based on 100 parts byweight of the diene rubber. In the case where a condensate of alkylphenol and sulfur chloride is contained as the vulcanizationacceleration auxiliary, its content is preferably 0.5 to 3.0 parts byweight based on 100 parts by weight of the diene rubber because a scorchtime tends to be shortened.

Examples of the vulcanization acceleration auxiliary other than thecitraconimide compound, the condensate of alkyl phenol and sulfurchloride, the organic thiosulfate compound, and the compound representedby the formula R¹—S—S-A-S—S—R² are, for instance,tetrabenzylthiuramdisulfide (TBZTD) available from Flexsys KabushikiKaisha and the like. However, TBZTD and the like accelerate the rate ofvulcanization more than enough due to a very high sulfur contentthereof, and therefore it is preferable not to compound them.

Also, an antioxidant, stearic acid, and zinc oxide which are generallyused in the rubber industry may be optionally compounded as thevulcanization acceleration auxiliaries as needed.

In the present invention, by compounding specified amounts of the dienerubber, the phenol resin and/or the modified phenol resin, sulfur, thespecific vulcanization accelerator, and the specific vulcanizationacceleration auxiliary, extrusion processability can be enhanced byoptimizing a rate of vulcanization during the vulcanization process, andalso rigidity and a low fuel consumption property can be enhanced.

Besides the aforementioned diene rubber, phenol resin and/or modifiedphenol resin, sulfur, vulcanization accelerator, and vulcanizationacceleration auxiliary, the rubber composition for a bead apex of thepresent invention may comprise compounding agents that have beenutilized generally in the rubber industry, for example, fillers such asa carbon black, silica, calcium carbonate, coal ash, clay and mica, asilane coupling agent, and N-cyclohexylthiophthalamide (CTP) accordingto necessity.

The complex modulus (E*) of the rubber composition for a bead apex ofthe present invention measured at 70° C. under the conditions of aninitial strain of 10% and a dynamic strain of 2% is preferably not lessthan 15 MPa, more preferably not less than 20 MPa. In the case where thecomplex modulus (E*) of the rubber composition for a bead apex of thepresent invention is less than 15 MPa, neither sufficient steeringstability nor good responsiveness to steering tends to be obtained. Atthe same time, the complex modulus (E*) of the rubber composition for abead apex of the present invention is preferably not more than 60 MPa,more preferably not more than 55 MPa. In the case where the complexmodulus (E*) of the rubber composition for a bead apex of the presentinvention is more than 60 MPa, there is a tendency that the elongationat break (E_(B)) is decreased and breakage occurs as a result of acollision with curbstones or deformation in mounting the tire to a rim.

When explaining by means of FIG. 1 which is a partial cross-sectionalview of the tire for a passenger car having a bead apex of the presentinvention, the bead apex 4 is a rubber portion arranged inside the tireclinch 2 and extending outwardly from the bead core 3 in the radialdirection, and functions to exhibit responsiveness to steering (steeringstability) since a torque is transmitted in the order of the rim 8, thetire clinch 2, the bead apex 4 and the inner liner 5.

The rubber composition for a bead apex of the present invention is arubber composition prepared in consideration of its use for a bead apexamong tire members because its viscoelasticity (rigidity) (E*) is highand its rolling resistance (tan δ) is low.

The tire (tire for a passenger car and tire for sports utility vehicle(SUV)) of the present invention is prepared by a usual method using therubber composition for a bead apex of the present invention. Morespecifically, the rubber composition of the present invention preparedby compounding the aforementioned compounding agents according tonecessity is extruded and processed into a shape of a bead apex of atire in an unvulcanized state, and then an unvulcanized tire is moldedin a usual manner on a tire molding machine. Then, the unvulcanized tireis subjected to heating and pressing in a vulcanizer to obtain a tire.

FIG. 3 is a diagrammatic view showing a ratio of a height of the beadapex of the present invention to a height of a section of a tire for apassenger car. FIG. 3 diagrammatically shows the height A of the beadapex 4 of the present invention to the height B of the section of a tirefor a passenger car.

FIG. 4 is a diagrammatic view showing a ratio of a height of the beadapex of the present invention to a height of a section of a tire for asports utility vehicle (SUV). FIG. 4 diagrammatically shows the height Cof the bead apex 14 of the present invention to the height D of thesection of a tire for a sports utility vehicle.

Tires for a sports utility vehicle (SUV) has properties such that whenparking in a cold district, permanent set is easily accumulated and flatspot vibration is easily caused, and therefore responsiveness to quicksteering is demanded. Accordingly it is preferable that the bead apexwhich is prepared by using the rubber composition for a bead apex of thepresent invention and has enhanced rigidity, durability and steeringstability is used for tires for a sports utility vehicle (SUV) in whicha ratio of a height of the bead apex to a height of a section is highand the bead apex is a relatively important member among tire members.

In the present invention, a ratio of a height of the bead apex to aheight of a section is a value of A/B in FIG. 3 in the case of a tirefor a passenger car, and a value of C/D in FIG. 4 in the case of a tirefor a sports utility vehicle (SUV).

FIG. 5 is a partial cross-sectional view of a tire for a passenger carin which the rubber composition for a bead apex of the present inventionis used for a strip apex of a tire for a passenger car.

The rubber composition for a bead apex of the present invention isexcellent in rigidity, and therefore as shown in FIG. 5, can be usedpreferably as a rubber composition for a strip apex 24 (for example, 1mm thick sheet) which is provided at the end of a bead apex 21 andcontacts with a sidewall 22 and an inner liner 23.

In addition, the rubber composition for a bead apex of the presentinvention can be used preferably as a rubber composition for an innerliner for a race tire.

EXAMPLES

Hereinafter, the present invention will be explained in detail based onExamples, but it should be understood that the present invention is notlimited thereto.

Various chemicals used in Examples and Comparative Examples will becollectively explained hereinafter.

Natural rubber (NR): RSS#3

Carbon black: SHOWBLACK N550 available from CABOT JAPAN Kabushiki Kaisha

Silica: 115GR available from Rhodia Japan, Ltd.

Silane coupling agent: Si69 (bis(3-triethoxysilylpropyl)tetrasulfide)available from Degussa GmbH

Modified phenol resin: PR12686 (phenol resin modified with cashew oil)available from Sumitomo Bakelite Co., Ltd.

Antioxidant: NOCRAC 6C(N-1,3-dimethylbutyl-N′-phenyl-p-phenylenediamine) available from OuchiShinko Chemical Industrial Co., Ltd.

Zinc oxide: available from Mitsui Mining & Smelting Co., Ltd.

Stearic acid: available from NOF Corporation

Sulfur: Crystex HSOT20 (insoluble sulfur containing 80% by weight ofsulfur and 20% by weight of oil, and in sulfur components, an insolublesulfur content is not less than 90% and a soluble sulfur content is notmore than 10%) available from Flexsys Kabushiki Kaisha

Vulcanization acceleration auxiliary (1): PERKALINK900(1,3-bis(citraconimidemethyl)benzene) available from Flexsys KabushikiKaisha and represented by the following formula:

Vulcanization acceleration auxiliary (2): TACKROL V200 (a condensate ofalkyl phenol and sulfur chloride, n: 0 to 10, X: 2, R: an alkyl group ofC₈H₁₇, sulfur content: 24% by weight) available from Taoka Chemical Co.,Ltd. and represented by the following formula:

Vulcanization acceleration auxiliary (3): 1,6-hexamethylene sodiumdithiosulfate dihydrate available from Flexsys Kabushiki Kaisha andrepresented by the following formula:

Vulcanization acceleration auxiliary (4): VULCUREN TRIAL PRODUCT KA9188(1,6-bis(N,N′-dibenzylthiocarbamoyldithio)hexane) available from BAYER.

Vulcanization accelerator (1): NOCCELER H (hexamethylenetetramine, HMT)available from Ouchi Shinko Chemical Industrial Co., Ltd.

Vulcanization accelerator (2): NOCCELER NS(N-tert-butyl-2-benzothiazolylsulfenamide) available from Ouchi ShinkoChemical Industrial Co., Ltd.

N-cyclohexylthiophthalamide (CTP): CTP available from Ouchi ShinkoChemical Industrial Co., Ltd.

Examples 1 to 6 and Comparative Examples 1 to 7

In accordance with the compounding prescriptions shown in Table 1,chemicals other than sulfur, the vulcanization acceleration auxiliaries(1) to (4), the vulcanization accelerators (1) and (2), andN-cyclohexylthiophthalamide (CTP) were kneaded in a Banbury mixer toprepare kneaded products. Then, sulfur, the vulcanization accelerationauxiliaries (1) to (4), the vulcanization accelerators (1) and (2), andCTP were added to the obtained kneaded products and were kneaded on anopen roll to prepare unvulcanized rubber compositions. Each of theobtained unvulcanized rubber compositions was press-vulcanized under thecondition of 170° C. for 12 minutes to prepare vulcanized rubbercompositions of Examples 1 to 6 and Comparative Examples 1 to 7.

As shown in Table 1, 2.5 parts by weight of the vulcanizationacceleration auxiliary (1) was contained in Example 1, 1.6 parts byweight of the vulcanization acceleration auxiliary (2) was contained inExample 2, 2.5 parts by weight of the vulcanization accelerationauxiliary (3) was contained in Example 3, 1.25 parts by weight of thevulcanization acceleration auxiliary (1) and 0.8 part by weight of thevulcanization acceleration auxiliary (2) were contained in Example 4,2.5 parts by weight of the vulcanization acceleration auxiliary (4) wascontained in Example 5, and 2.5 parts by weight of the vulcanizationacceleration auxiliary (1) was contained in Example 6 under thecondition that neither silica nor silane coupling agent was compounded.These contents were so adjusted that the same level of crosslinkingdensity could be obtained. Here the same level of crosslinking densityreferred to in the present invention means that crosslinking densitiesof the obtained rubber compositions are the same and when thecrosslinking densities are the same, properties such as a swellingdegree (swell), hardness (Hs) and complex modulus (E*) are also nearlythe same.

(Initial Vulcanization Rate)

An initial vulcanization rate is measured according to JIS K 6300(Unvulcanized rubber—Physical properties—Part. 1: Method for measuringviscosity and scorch time using a Mooney viscometer).

In “Mooney scorch test” according to JIS K 6300, using a L-shaped rotor,a period of time (Mooney scorch time, referred to as t₁₀) required forelevating a Mooney viscosity of the above-mentioned unvulcanized rubbercomposition (vulcanizable rubber composition) to 10M (M represents aMooney viscosity) is determined at a measuring temperature of 130° C.For measuring the Mooney scorch time (minute), a M-time curve (unit:minute) showing a relation between the Mooney viscosity and the time isprepared.

It is necessary that the Mooney scorch time is not less than 7.0 minutesfor the reasons that it is necessary to charge the rubber sheet ofnormal temperature in an extruder, heat it to increase flowability andthen discharge it in a desired shape without causing the partial scorch,and also for the reason that the rubber temperature at dischargingreaches around 130° C., and a longer residence time in the rubberextruder can be secured. When the Mooney scorch time is 7.0 minutes ormore, unvulcanized rubber compositions are processable. In addition, itis necessary that the Mooney scorch time (minute) is not more than 30minutes for the reason that insufficient vulcanization does not occurwhen vulcanizing in a mold after molding of an unvulcanized rubber.

(Viscoelasticity Test)

Using a viscoelasticity spectrometer VES (manufactured by IwamotoSeisakusyo Kabushiki Kaisha), a complex modulus (E*) and a loss tangent(tan δ) of the respective vulcanized rubber compositions are measured at70° C. under the conditions of an initial strain of 10% and a dynamicstrain of 2%. The larger the E* is, the higher the rigidity is and thussteering stability is superior. The smaller the tan δ is, the morepreferable the low fuel consumption property is.

(Steering Stability)

The aforementioned unvulcanized rubber compositions are formed into ashape of a bead apex, and then press-vulcanized together with other tiremembers under the condition of 170° C. for 12 minutes to produce tiresfor sports utility vehicles (tires for SUVs, tire size: P265/65R17110S).

In addition, tires for passenger cars (tire size: 195/65R15 91S) areproduced by forming the aforementioned unvulcanized rubber compositionsinto a shape of a bead apex, and then press-vulcanizing together withother tire members under the condition of 170° C. for 12 minutes.

The tires for SUV thus produced are mounted on a sports utility vehicle(SUV), and the tires for a passenger car thus produced are mounted on apassenger car (NOAH produced by TOYOTA MOTOR CORPORATION), and the SUVand the passenger car are run on a test course to perform sensory teststo evaluate steering stability and responsiveness to steering. Steeringstability and responsiveness to steering are evaluated on the basis of 6points at maximum based on the steering stability and responsiveness tosteering of Comparative Example 1 of 5. The larger the point is, themore excellent the steering stability and responsiveness to steeringare. The tires of the point 5⁺ represent that their steering stabilityand responsiveness to steering are better than the tires of the point 5but inferior to the tires of the point 6.

(Rolling Resistance)

Using a rolling resistance tester manufactured by Kobe Steel, Ltd., thetires for SUV are run under the conditions of a load of 30 N, a tireinner pressure of 200 kPa and a speed of 80 km/hr to measure rollingresistances. The rate (%) of change in rolling resistance of eachcomposition is expressed by an index based on the rolling resistance ofComparative Example 1 (±0) by calculating according to the followingequation. The smaller the rate of change in rolling resistance is, themore the rolling resistance is decreased and thus it is more preferable.Specifically, a negative value is preferable.

(Rate of change in rolling resistance)=(Rolling resistance of each ofthe compositions−Rolling resistance of Comparative Example 1)/(Rollingresistance of Comparative Example 1)×100

(Indoor Load Durability Test on Drum)

A tire for SUV is run on a drum at a speed of 20 km/h by applying, tothe tire, a load of 230% of a maximum load (maximum inner pressurecondition) as defined in JIS, and a running distance until swelling at abead part is generated is measured. The running distance of ComparativeExample 1 is expressed by an index of 100, and each of the runningdistances of the other rubber compositions is represented by an index(indoor load durability index). The larger the value is, the superiorthe durability is.

(Indoor load durability index)=(Running distance of eachcompounding)÷(Running distance of Comparative Example 1)×100

Evaluation results of the aforementioned tests are shown in Tables 1 and2.

TABLE 1 Example 1 2 3 4 5 6 Amounts (part by weight) NR 100 100 100 100100 100 Carbon black 50 50 50 50 50  55 Silica 10 10 10 10 10 — Silanecoupling agent 1.0 1.0 1.0 1.0 1.0 — Modified phenol resin 18 18 18 1818  18 Antioxidant 1.5 1.5 1.5 1.5 1.5   1.5 Zinc oxide 5 5 5 5 5  5Stearic acid 3 3 3 3 3  3 Insoluble sulfur (containing 20% of oilcontent) 7.5 7.5 7.5 7.5 7.5  7.5 Vulcanization acceleration auxiliary(1) 2.5 — — 1.25 —  2.5 Vulcanization acceleration auxiliary (2) — 1.6 —0.8 — — Vulcanization acceleration auxiliary (3) — — 2.5 — — —Vulcanization acceleration auxiliary (4) — — — — 2.5 — Vulcanizationaccelerator (1) 1.8 1.8 1.8 1.8 1.8  1.8 Vulcanization accelerator (2)3.7 3.7 3.7 3.7 3.7  3.7 CTP 0.6 0.6 0.6 0.6 0.6  0.6 Evaluation ResultsScorch time (minute) 8.0 7.1 7.3 7.5 7.2 7.5 Viscoelasticity test E*(MPa) 50 51   50   50   50   52   tan δ 0.11  0.105  0.115  0.113  0.116 0.11 Steering stability Tire for SUV 6 6   5⁺  5⁺  5⁺  6   Tire forpassenger car 5⁺ 5⁺  5   5   5   5   Rate of change in rollingresistance (%) −2.0 −2.2  −1.8  −1.8  −1.7  −2.0  Indoor load durabilityindex 125 130    115    115    115    120   

TABLE 2 Comparative Example 1 2 3 4 5 6 7 Amounts (part by weight) NR100 100 100 100 100 100 100 Carbon black 50 50 50 50 50 50 50 Silica 1010 10 10 10 10 10 Silane coupling agent 1.0 1.0 1.0 1.0 1.0 1.0 1.0Modified phenol resin 18 18 22 18 18 18 18 Antioxidant 1.5 1.5 1.5 1.51.5 1.5 1.5 Zinc oxide 5 5 5 5 5 5 5 Stearic acid 3 3 3 3 3 3 3Insoluble sulfur (containing 20% of oil content) 7.5 9.0 7.5 7.5 6.256.25 9.0 Vulcanization acceleration auxiliary (1) — — — — 2.5 — 2.5Vulcanization acceleration auxiliary (2) — — — 1.6 — 1.6 — Vulcanizationacceleration auxiliary (3) — — — — — — — Vulcanization accelerationauxiliary (4) — — — — — — — Vulcanization accelerator (1) 1.8 1.8 2.20.5 1.8 1.8 1.8 Vulcanization accelerator (2) 5.0 3.7 3.7 3.7 3.7 3.73.7 CTP 0.6 0.6 0.6 0.6 0.6 0.6 0.6 Evaluation Results Scorch time(minute) 5.4 9.2 8.0 8.2 9.0 8.2 6.5 Viscoelasticity test E* (MPa) 42 4450 32 32 33 58 tan δ 0.14 0.15 0.15 0.14 0.12 0.112 0.101 Steeringstability Tire for SUV 5 5 5⁺ 3 3 3 6 Tire for passenger car 5 4⁺ 5 4 44 5+ Rate of change in rolling resistance (%) ±0 ±0 +0.5 +0.4 −1.2 −1.7−2.4 Indoor load durability index 100 95 95 105 120 125 80stability and reducing rolling resistance and a tire for a sportsutility vehicle (SUV) having further enhanced durability.

1. A bead apex composed of a rubber composition for a bead apex, saidrubber composition comprising: a diene rubber, and, based upon 100 partsby weight of said diene rubber; 5 to 25 parts by weight of a phenolresin and/or a modified phenol resin; 5.1 to 7.0 parts by weight ofsulfur; 1.0 to 2.5 parts by weight of hexamethylenetetramine; 2.0 to 5.0parts by weight of a sulfenamide vulcanization accelerator and/or athiazole vulcanization accelerator; and 0.1 to 5 parts by weight of acitraconimide compound, wherein the rubber composition has a Mooneyscorch time of not less than 7.0 minutes and not more than 30 minutes.2. The bead apex of claim 1, wherein the total amount ofhexamethylenetetramine and sulfenamide vulcanization accelerator and/orthiazole vulcanization accelerator is 3.5 to 7.5 parts by weight basedon 100 parts by weight of the diene rubber.
 3. The bead apex of claim 1,wherein the citraconimide compound is1,3-bis(citraconimidemethyl)benzene represented by the followingchemical formula:


4. The bead apex of claim 1, wherein the diene rubber is a naturalrubber.
 5. A tire having the bead apex of claim
 1. 6. A tire for asports utility vehicle having the bead apex of claim 1.